The strength of steel sheets used in the automobile industry has been constantly increased to allow automobiles satisfying environmental regulations and having high fuel efficiency to be manufactured. However, it is difficult to form high-strength steel strips into desired shapes, and the load required in processes for correcting the shapes of high-strength steel strips is high. In addition, even in the case that high-strength steel is processed using a shape-correcting process, the shape of the high-strength steel may not be easily corrected. Due to this reason, after increasing the temperature of a steel strip, the shape of the steel strip may be corrected. In this case, if the strip is corrected within a warm working temperature range without using a roll cooling device, work rolls may be heated by the strip and expanded to have a large crown shape known as a thermal crown, and thus, uncontrollable severe wave shapes may be formed in the center region of the strip, rendering the strip useless as a product.
Moreover, in a correcting process, a large amount of scale formed on a steel strip may be separated therefrom, and when cooling water is applied to cool rolls, the separated scale may scatter to cause secondary surface defects, thereby making it difficult to use cooling water.
In the related art, cooling is not performed due to this reason, and thus, a shape-correcting process is restrictively performed at a strip temperature of about 50° C. In this case, however, the effect of the shape-correcting process is low, and the number of steel strips that can be continuously rolled is limited. In addition, it is necessary to supply a warm-rolling workpiece immediately before rolls are changed and to change the rolls after warm rolling.
Patent Document 1 (please refer to FIG. 1) discloses a technique for economically manufacturing high-strength strips by unwinding a hot-rolled strip 3, having a temperature higher than a surrounding temperature, from a pay-off reel 4, correcting the strip 3 using a correcting machine 5, and passing the strip 3 through a furnace 6 to anneal the strip 3. Patent Document 2 discloses a technique for performing a skin pass milling process on high-tension steel within a warm-working temperature range of 60° C. to 120° C.
However, Patent Documents 1 and 2 only state the effects of a shape-correcting process or a skin pass milling process performed within a warm-working temperature range higher than room temperature but do not state how the shape-correcting process is practically performed within the warm-working temperature range. That is, Patent Documents 1 and 2 do not disclose the possibility of shape errors caused by deformation of rolls heated when a strip is merely corrected at a temperature higher than room temperature.
(Patent Document 1) KR10-1153732 B
(Patent Document 2) JPH10-005809 A